Is my battery or alternator going?

[teambeechstreet]

Two nights ago, I started up my beater 87 Civic hatch and the Battery light came on. Now it will come on sometimes when the car is started, sometimes not for a while down the road, sometimes not at all. In addition, it sometimes flickers rapidly (not blinking, looks like a fluorescent bulb about to go out), then goes off for a while, and comes back on. Turn signals, headlights, and interior lights wouldnt turn on.

All connections to alternator/battery/ignition/etc checked and all good. I am not sure if the alternator is failing, as I drove the car for over an hour with the light going on and off (mostly on).

I had the battery charged last night to 100% and the battery light turned off and everything worked fine and I left the car at home and took my 04 Civic to work. This morning, I went to start the 87 and it started up fine, but the battery light is now back on and the headlights are very dim. The car also ran very rough. I checked the battery again and it is only at 70% charge.

So does this sound like the battery has gone bad or is it the charging system? Thanks for ur help in advance.

 

[Virtuoso]

Alternator.

 

[johnd]

A voltmeter would be a big help here. If the voltage running isn't 13.8-14.2V, then the alternator isn't working right. If it is and the battery isn't charging, it's the battery. I'm betting that it is the alternator.

 

[teambeechstreet]

I forgot to mention that the alternator was replaced about 10K miles ago. Could it still be the culprit? And thanks for the voltmeter idea, I'm gonna go grab mine and check it out.

 

[Virtuoso]

Rebuilt alternator? Lots of them are crap out of the box

I hope mine lasts awhile! *crosses fingers*

 

[mossad]

Yes, it could be the culprit. There is no guarantee on how long they will last, especially if it is a refurbished alterntator which one would buy frm Autozone. Once the car is started, the battery really has nothing more to do with the electrical system once the alternator is doing its job. The only job of the battery is to assist in starting the car and/or operate an additional sound equipment one might add to their ride.

 

[teambeechstreet]

thanks for the replies, guyz. under high idle with no "accessories" on it read at 12.10 volts DC. At high idle with headlights, radio, turn signals, and heater it read 11.9 volts and at normal idle with no accessories it read 12.5 volts. Sounds like the alternator is not up to snuff. The way the alternator is jammed in that engine bay behind a maze of wires and hoses, looks like I'm gonna be driving the 04 Civic for awhile until I have some free time.

 

[kenw]

voltmeters only tell part of the story: volts.

The alternator must also output sufficient amps to charge the battery and power all the devices. If one output transistor goes bad, you can in some cases get the proper voltage reading but once the current drain increases, the volts will drop.

You need to check the alternator output under various loads (current, amps) to verify that it is the problem.

A decent parts house can check it under load.

 

[kenw]

we evidently cross-posted simultaneously: your results indicate clearly a bad alternator. You basically varied the load and saw the volts change, so you've done it right.

Now if you wait too long you WILL buy an new battery as well, so don't wait to get it replaced.

 

[teambeechstreet]

quote:

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Originally posted by kenw:
we evidently cross-posted simultaneously: your results indicate clearly a bad alternator. You basically varied the load and saw the volts change, so you've done it right.

Now if you wait too long you WILL buy an new battery as well, so don't wait to get it replaced.

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thanks. the 87 civic will be sitting in the driveway while i drive the 04 Civic until I can find time to swap out the alternator. gosh darn, and the whole purpose of buying the 87 was to keep mileage off my 04. Oh well...

 

[brianl703]

quote:

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Originally posted by mossad:
Once the car is started, the battery really has nothing more to do with the electrical system once the alternator is doing its job.

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Once the car is started, the battery filters the output of the alternator (which is rectified, pulsing DC) as well as to supply electrical demand during that short period of time between when a high-load electrical device turns on and the voltage regulator can turn up the field coil current.

 

[teambeechstreet]

well i got the alternator out. was a lot easier than I thought it was going to be